It is known that lubricating rolling cylinders in iron and steel mill is often achieved by spraying (or vaporising) an emulsion, which is a suspension of droplets of oil in water, with an oil concentration typically varying between 0.3 and 2%. It may either be a stable emulsion or an unstable emulsion prepared on the line.
In general, the emulsion is directly applied to the working rollers by means of a spray device located after the scrapers with a view to applying the oil onto a dry surface. This application method ensures better distribution of the oil and as a result contributes to reducing oil consumption.
The point of good lubrication of the working rollers is not simply restricted to the problem of the performance of the rollers (surface deterioration) but is also associated with the rolling forces and torques to be applied, and hence with the required electricity consumption. The need to have efficient lubrication is even more acute when using HSS-type rollers characterised by a higher friction coefficient than traditional rollers, with harder, thinner steels, in conditions of increased production and surface quality imposed by stricter clients.
Among the lubrication conditions, the friction coefficient depends on numerous factors such as the application technology used, the quantity of oil, the nature of the oil, the flow rate and concentration of the oil emulsion, the surface temperature of the strip or of the cylinders, the nature and state of the rollers (roughness, deterioration, scale, thickness of the water film, etc.), the force and speed of rolling, the reduction, the grade and the surface condition of the product, etc. Thus, the lubrication efficiency may be very different from one rolling mill to the next and from one stand to the next in the same mill.
In the laboratory, it was established that the lubrication efficiency depends on the technology for delivering the lubricant, on the nature of the lubricant (mineral, ester-based, etc.) and on the quantity of lubricant implemented. Satisfactory results are obtained by spraying oil over surface distributions (“plate-out”) varying from 0.1 to 1 g/m2.
Conventionally, the device for spraying the oil emulsion is either a venturi nozzle, the oil being sucked by the low pressure created by the water moving in the main tube, or a device with conventional flat nozzles for injecting a stable emulsion, for example a static tube mixer, where the oil is injected into a zone of the tube where the shear (velocity gradient) is increased thanks to the presence of “obstacles”. The venturi nozzle or the static tube mixer is combined with a series of jets, the number of which is selected depending on the width of the strip to be lubricated (three to seven jets for a strip of up to 2 meters wide).
Document EP-A-1 193 004 describes a lubrication method for cold rolling comprising the stages of: providing a rolling oil emulsion using a first oil delivery device in the form of an emulsion delivered to a cylinder and a steel strip in a closed circuit as well as a second emulsion delivery device only on the front and back surfaces of the steel strip. In the second device, the rolling oil is added to an emulsifying agent of the same type and concentration as those used for the first device, with a control of the average particle size to ensure that they are larger than in the first device. The emulsion produced by the second device that did not adhere to the strip is recovered at the same time as the emulsion produced by the first device.
Document WO-A-03/002 277 discloses an installation for cooling and lubricating working rollers in a rolling stand, comprising a cooling water spray bar and a separate spray bar for lubrication oil, for an oil/air, oil/water or oil/air/water mixture, or even for grease.
Document WO-A-03/000 437 discloses an installation and a method for lubricating mill rollers in which an emulsion of oil in water in adjustable proportions is homogeneously prepared in a mixer and delivered to various spray zones, the distribution of which is variable in width. Each zone corresponds to a row of nozzles, each nozzle being controlled by at least one relay valve.
Document JP-A-2001/179 313 discloses a device for applying lubricant, either in the form of undiluted oil or of an emulsion, with a lattice structure allowing even adhesion of the lubricant to a working roller in a rolling stand.
U.S. Pat. No. 3,933,660 proposes a reducing lubrication oil for the hot rolling of copper and its alloys comprising 1,000 parts by weight of water, 6 to 200 parts by weight of anionic surface activator of a carbolic, sulphate or phosphate acid type and 0.8 to 200 parts by weight of at least one compound comprising a hydroxyl group of an alcohol, glycol alkylene or glycol ether type. The rolling oil gives copper and its alloys lubrication, the capacity to remove an oxide film and a capacity to prevent the formation of an oxide film by spraying between the rolling cylinder and the strip to be hot rolled.
Document JP-A-2003/129 079 discloses a lubricant mixture for the plastic working of a metal comprising a carboxylate, a metal acid phosphate or a metal alkylphosphonate.
Document JP-A-55 151 093 describes a lubrication method for the cold rolling of a strip covered with a polar organic compound, where an emulsion of oil and of a mixture comprising a polar organic mixture such as stearic acid is sprayed. The surplus oil emulsion is removed by drying after an oil layer adsorbed to the surface of the strip is formed.
Document WO-A-2005/071 050 discloses a self-emulsifying lubricant for working with metals obtained by maleation of vegetable or animal triglyceride oil.
When used on an industrial scale, the main drawbacks inherent in these devices for spraying an oil emulsion are the following:                the oil-water interaction, which depends on the type of oil and on the “quality” of the water, determines the time for the oil to adhere to the roller as well as the quantity that adheres. The efficiency of the devices used is therefore not easily predicted;        the performance of the unstable emulsions used is strongly correlated with the ability to maintain good dispersion of the oil in the water or, in other words, with the value and stability over time of the shearing rate obtained, which is correlated with the velocity gradient, which depends on both the length and diameter of the tube used and on the emulsion flow rate. In fact, it is difficult to maintain this stability between the static tube mixer and the various jets; in other words, it is difficult to ensure the controllability of the friction coefficient obtained;        bringing the oil and water into contact usually leads to reactions that form a hard, adhesive polymer phase, blocking the feeder pipes and the jets.        
To overcome these drawbacks, the Applicant already proposed in document EP-A-1 512 469 a method and an installation for the in-line lubrication of hot rolling cylinders allowing to maintain the friction constant or in any event under control on an industrial scale, to increase the adhesion efficiency of the lubrication oil to the cylinders and to increase the evenness of distribution of the oil over the cylinders.
According to this method, lubrication is achieved by spraying or atomising a lubricant or a mixture of lubricants in the vicinity of the gap of the working cylinders by means of a closed chamber equipped:                with a means for forming a cloud of lubricant droplets having a size that is smaller than 700 μm, and preferably smaller than 200 μm;        with a diaphragm with an adjustable opening connected to said means and positioned on a front surface of the chamber and        with a device for recovering surplus lubricant on the part of the diaphragm inside the chamber and on the internal walls of the chamber.        